350 
Fishery Bulletin 95(2), 1 997 
its life history aspects, such as maximum size and 
food preferences. One particular aspect of guitarfish 
behavior is that large numbers of them are often 
found in shallow embayments, such as Elkhorn 
Slough and Mugu Lagoon, California, and Almejas 
Bay, Baja California, Mexico. In these areas, they 
are easily captured with a seine net and are thus 
particularly susceptible to fishing pressure. 
Because elasmobranchs tend to be exploited be- 
fore regulatory measures are in effect (Pratt and 
Casey, 1990), it is necessary to determine age and 
growth relationships and size at sexual maturity of 
R. pi'oductus prior to increases in fishing pressure. 
The results of this study provide basic information 
for management of guitarfish, should it become more 
popular as a food item. 
We have incorporated the following methods of age 
determination into this study of the age, growth, and 
sexual maturity of guitarfish: 1) a laboratory analy- 
sis of the vertebral bands and their outer margin 
state (translucent or opaque) in order to assign ages 
to individuals; 2) a study of growth in captivity to 
verify estimated growth from the laboratory analy- 
sis; and 3) a determination of age at sexual matu- 
rity. The main focus of this age and growth study is 
based on an examination of vertebral centra and their 
use in ageing guitarfish. 
Methods 
Age and growth 
Seventy-five guitarfish were collected between No- 
vember 1988 and January 1991 from the waters be- 
tween Seal and Redondo Beaches, California (Fig. 
1). Guitarfish were captured by hook and line, gill 
net, otter trawl, long line, or beach seine, and then 
frozen. Lengths were measured with a tape measure 
to the nearest centimeter over the contour of the dor- 
sal portion of the guitarfish and included total length 
(TL), disc width (DW), first dorsal fin length (ID), 
and second dorsal fin length (2D) (Fig. 2). The con- 
tour measurement over the dorsal portion provided 
a more precise measurement of the first and second 
dorsal fin lengths. This method will increase the to- 
tal length measurement and should be taken into 
consideration if comparisons are made with lengths 
of guitarfish in this study. 
The only portion of the guitarfish that is available 
in fish markets is the trunk and tail or loin region, 
which includes the two dorsal fins. Therefore, we 
included the measurement of the distance from the 
origin of the first dorsal fin to the origin of the sec- 
ond dorsal fin (2D) to facilitate future predictions of 
Study sites where guitarfish, Rhinobatos productus, were col- 
lected along the coast of southern California. A = Redondo Beach, 
B = Palos Verdes, C = San Pedro, D = Long Beach, E =Belmont 
Shores, and F = Seal Beach. 
total length from market fish. Damp weight was 
measured for all guitarfish with a spring balance. 
Ten vertebrae were removed from each guitarfish just 
anterior to the first dorsal fin for analysis. The larger 
vertebrae were located just posterior to the eyes; 
however, they were not used because removal of these 
vertebrae would have interfered with dissection of 
the female reproductive tract. Each guitarfish was 
assigned a code number and this became the only 
identifying feature for each guitarfish for the remain- 
der of the study. Vertebrae were cleaned by placing 
them in a dermestid beetle colony. The beetles con- 
sumed almost all muscle and connective tissue; the 
only remaining tissue was a cone-shaped membrane 
(membrane elastica externa) on the centrum that was 
easily removed from the dry vertebrae with fine for- 
ceps. Cleaned and dried vertebral centra were viewed 
whole with a Wilde dissecting scope with transmit- 
ted light within a dark field. Ten vertebrae from each 
guitarfish were examined to determine consistency 
of band formation within an individual. If all verte- 
brae for an individual guitarfish contained the same 
number of bands, then two of those vertebrae were 
used for three separate readings. Those having vari- 
able band counts or unreadable vertebrae among the 
ten vertebrae were discarded. Opaque bands present 
beyond the birth mark were counted (Fig. 3). Rings 
within bands were not always discernible as sepa- 
rate rings; therefore, bands were determined to be 
the most useful increment. The birth mark was de- 
fined here as the centermost opaque portion (first 
band) of the centra. It was present in the smallest of 
the guitarfish and was in the same position in all 
larger specimens. This birth mark is similar in place- 
